To prepare the chicken, season the flour with salt and pepper. Slice the chicken breasts to 1/4-inch-thick medallions. Coat with the flour and tap off any excess.
Next, in a saucepan over medium heat, add enough oil to coat the bottom of the pan and allow to warm. After warming, add the chicken and cook until browned, 2 to 3 minutes. Flip and repeat the process. After cooking all the chicken, hold warm covered on a plate. Add the marsala to the pan over medium-high heat. This will deglaze the pan. After deglazing, reduce the wine by three-fourths volume so 2 ounces remain. Then add the mushrooms. Stir the mushrooms, scraping the sides, then finish with the veal stock. Allow the stock to warm and thicken for 4 to 5 minutes, and then remove from the heat. Stir in the butter and finish by spooning over the cooked chicken. Garnish with fresh herbs or cheese if desired.